Rare-earth (RE) doped optical fibres have shown great potential for a number of applications including fibre lasers, amplifiers and sensors. In contrast to germanosilicate core silica glass optical fibres which are used for transmission of signal over long distances, the presence of RE ions in the core makes the fibres optically active due to the characteristic emission of the RE when pumped at suitable wavelengths. Lasing and amplification have been demonstrated at several wavelengths with the incorporation of the various rare-earths.
While the erbium doped fibre remains the most important for telecommunication applications, fibres doped with other rare earths are gaining importance mostly for development of laser sources from visible to mid infrared regions. The Yb doped and Er/Yb codoped fibres are of special mention in this respect for development of high power fibre lasers generating short single-frequency pulses for communication and also for industrial and medical applications.
The erbium doped fiber which is the active medium of an EDFA (erbium doped fiber amplifier) has been an enabling technology for optical networks operating in the third telecommunication window between 1530 and 1610 nm. EDFA can simultaneously amplify several optical channels in a single fibre which has enabled the implementation of DWDM (dense wavelength division multiplexing) technology with the potential of increasing the bandwidth of long distance transmission systems from Gb/s to Tb/s ranges. It exhibits high gain, large bandwidth, low noise, polarisation insensitive gain, substantially reduced cross talk problems and low insertion losses at the operating wavelengths. The deployment of EDFA has spurred a tremendous growth in advanced telecommunication systems replacing the conventional optoelectronic repeaters.
Reference may be made to Townsend J. E., Poole S. B., and Payne D. N., Electronics Letters, Vol. 23 (1987) p-329, ‘Solution-doping technique for fabrication of rare-earth-doped optical fibre’ wherein the MCVD process is used to fabricate the preform with a step index profile and desired core-clad structure while solution doping is adopted for incorporation of the active ion. The steps involved in the process are as follows:                A conventional cladding doped with P2O5 and F is deposited within a high silica glass substrate tube to develop matched clad or depressed clad type structure.        The core layers of predetermined composition containing index raising dopant like GeO2 are deposited at a lower temperature to form unsintered porous soot.        The tube with the deposit is immersed into an aqueous solution of the dopant precursor (typical concentration 0.1 M) up to 1 hour. Any soluble form of the dopant ion is suitable for preparation of the solution although rare earth halides have been mostly used.        Following immersion, the tube is rinsed with acetone and remounted on lathe.        The core layer containing the RE is dehydrated and sintered to produce a clear glassy layer. Dehydration is carried out a temperature of 600° C. by using chlorine. The level of OH− is reduced below 1 ppm using Cl2/O2 ratio of 5:2 provided the drying time exceeds 30 min.        Collapsing in the usual manner to produce a solid glass rod called preform.        Fibre drawing is conventional.        
Another reference may be made to J. Kirchhof, S. Unger, L. Grau, A. Funke & P.Kleinert, Crystal Research Technology, Vol .25, No. 2, 1990, pp. K29-K34, ‘A New MCVD Technique for Increased Efficiency of Dopant Incorporation in Optical Fibre Fabrication’, wherein the “alternating deposition” technique has been applied to improve the efficiency of deposition and dopant incorporation. In the above process, each single layer is built up by means of two-torch process:                1) formation of an unconsolidated SiO2—layer by opposite movement of gas and torch (or by parallel movement at low temperatures), and        2) consolidation of the layer under the influence of a gaseous mixture of oxygen and dopant source without further SiO2 deposition.Yet another reference may be made to I. Kasik, V. Matejec, J. Kanka, P. Honzatko, Pure Applied Optics, Vol. 7, 1998, pp.457-465, ‘Properties and fabrication of ytterbium-erbium co-doped silica fibres for high-power fibre lasers’, wherein the backward deposition was followed for depositing the porous core layer.        
Still another reference may be made to V. Matejec, I. Kasik, D. Berkova, M. Hayer, M. Chomat, Z. Berka, A. Langrova J. Kanka, P. Honzatko, Ceramics-Silikaty, Vol. 45, No. 2, 2001, pp.62-69, ‘Properties of optical fiber performs prepared by inner coating of substrate tubes’, wherein the porous core layer composed of SiO2 and P2O5 was deposited on the inner wall of the substrate tube by the reverse deposition MCVD technique at a temperature of 1400° C. The layer was presintered in order to fix the deposit. The deposited layer was soaked with an aqueous solution of AlCl3 and rare-earth chloride salts. The soaked layer was sintered at a temperature of 1000-1600° C. in the POCl3 atmosphere.
One more reference may be made to European Patent No. EP 1 043 281 A1 (2000) by Tankala, Kanisha. Sturbridge Mass. (US), ‘Method of fabricating preforms doped with rare earth metal for optical fibers’ wherein a porous silica soot layer was deposited on inside of a silica-based substrate tube. The porous silica soot layer was immersed in an impregnation solution having rare earth elements and other codopants also. The porous silica soot layer was subsequently sintered into a glass layer. A mixture of codopant precursor and oxygen was flown over the porous silica soot layer during said sintering step. Then the tube was collapsed to make a preform.
One another reference may be made to Guillaume G. Vienne, Julie E. Caplen, Liang Dong, John D. Minelly, Johan Nilsson, and David N. Payne, Journal of Lightwave Technology, Vol. 16, No. 11, 1998, pp. 1990-2001, ‘Fabrication and Characterization of Yb3+: Er3+ Phosphosilicate Fibers for Lasers’, wherein the backward deposition was adopted to deposit the porous core layer and presintering the above layer. For the deposition of silicate or germanosilicate porous layers the burner and the reactants were co propagating. The viscosity of the soot particles was high enough to prevent them from fusing when the burner passes over, thus leaving a porous layer. On the other hand, a phosphosilicate porous layer, also called “frit”, containing higher than 5 mol % phosphorous was easily fused by the burner at the deposition stage. The problem was solved by translating the burner in the opposite direction to the reactants i.e. by the backward deposition technique. There was no flow of any codopants in the presintering stage. The presintered frit was immersed in a solution of deionised water or methanol in which high purity rare-earth chlorides had been dissolved. After the solution doping the tube was dried for a certain period. Then the frit was fused by heating to around 1500° C. and the tube was collapsed to make a perform from which fibre was drawn.
A further reference may be made to DiGiovanni D. J., SPIE Vol. 1373 (1990) p-2 “Fabrication of rare-earth-doped optical fibre’ wherein the substrate tube with the porous core layer is soaked in an aqueous or alcoholic solution containing a nitrate or chloride of the desired RE ion. The tube is drained, dried and remounted on lathe. The dehydration is carried out by flowing dry chlorine through the tube at about 900° C. for an hour. After dehydration, the layer is sintered and the tube is collapsed to be drawn to fibre.
Another reference may be made to Ainslie B. J., Craig S. P., Davey S. T., and Wakefield B., Material Letters, Vol. 6, (1988) p-139, “The fabrication, assessment and optical properties of high-concentration Nd3+ and Er3+ doped silica based fibres” wherein optical fibres based on Al2O3—P2O5—SiO2 host glass doped with high concentrations of Nd3+ and Er3+ have been fabricated by solution method and quantified. Following the deposition of cladding layers P2O5 doped silica soot is deposited at lower temperature. The prepared tubes are soaked in an alcoholic solution of 1M Al(NO3)3+various concentrations of ErCl3 and NdCl3 for 1 hour. The tubes are subsequently blown dry and collapsed to make preforms in the usual way. Al is said to be a key component in producing high RE concentrations in the core centre without clustering effect. It is further disclosed that Al and RE profile lock together in some way which retards the volatility of RE ion. The dip at the core centre is observed both for P and Ge.
Yet another reference may be made to U.S. Pat. No. 5,474,588 (1995) by Tanaka, D. et. al., ‘Solution doping of a silica with erbium, aluminium and phosphorus to form an optical fiber’ wherein a manufacturing method for Er doped silica is described in which silica glass soot is deposited on a seed rod (VAD apparatus) to form a porous soot preform, dipping the said preform into an ethanol solution containing an erbium compound, an aluminium compound and a phosphoric ester, and desiccating said preform to form Er, Al and P containing soot preform. The desiccation is carried out for a period of 24-240 hours at a temperature of 60°-70° C. in an atmosphere of nitrogen gas or inert gas. This desiccated soot preform is heated and dehydrated for a period of 2.5-3.5 hours at a temperature of 950°-1050° C. in an atmosphere of helium gas containing 0.25 to 0.35% chlorine gas and further heated for a period of 3-5 hours at a temperature of 1400°-1600° C. to render it transparent, thereby forming an erbium doped glass preform. The segregation of AlCl3 in the preform formation process is suppressed due to the presence of phosphorus and as a result the doping concentration of Al ions can be set to a high level (>3 wt %). The dopant concentration and component ratio of Er, Al and P ions are claimed to be extremely accurate and homogeneous in the radial as well as in longitudinal directions.
The drawbacks of the above mentioned processes are as follows:    1. The porous soot layer deposition by following forward pass method, even for germanosilicate composition, leads to variation in composition and soot density along the length of the tube due to simultaneous presintering during the deposition pass and excessive temperature sensitivity.    2. Consolidation of the soaked soot layer containing the RE along with codopant like P2O5, GeO2 or such refractive index modifiers in presence of a gaseous mixture of O2 and POCl3, GeCl4 etc. requires high flow of the dopant halides to maintain such atmosphere inside the tube.    3. In case of soot layer containing high concentration of GeO2 the sintering in presence of GeCl4 leads to loss of considerable amount of GeCl4 and increase in cost of the preform/fibre.    4. Controlling the proportion of POCl3, GeCl4 etc. during consolidation with the input gases like oxygen becomes critical in order to achieve the desired composition as well as the properties like numerical aperture of the fibre.    5. There is possibility of change in composition of the porous soot layer due to evaporation of the dopants like P2O5, GeO2 etc. during oxidation and drying steps prior to consolidation.    6. The diffusion of the dopant halides like POCl3, GeCl4 etc through the entire soot deposit during the quick sintering step is difficult because of very short interaction time. Depending on the thickness of the porous soot layer the dopants are mostly confined to a region of the consolidated layer leading to variation in composition and degradation in the optical properties of the fibre.    7. The concentration of the dopants like P2O5, GeO2 etc. in the consolidated glass layer is very much dependant on the temperature during consolidation because of the complicated process mechanism.    8. The reproducibility and reliability of the process decrease due to the reasons stated above.